1. Field of the Invention
The present invention relates to an ophthalmic measuring apparatus capable of measuring the corneal shape and the eye refractive power in a single unit.
2. Description of the Prior Art
At an examination of eye refractive power, measurement of the corneal shape is usually carried out in addition to the measurement of refractive power, for the purpose of examining the presence of astigmatism and the axis and degree thereof. Particularly because of the recent popularity of contact lenses, the examination of corneal shape has become more important for selecting the base curve of the contact lenses.
Heretofore, at such examination for prescribing a contact lens, it has been customary to measure the corneal shape of the examinee with an instrument called ophthalmometer or keratometer thereby selecting the base curve and determining the degree of corneal astigmatism, then to determine the full refractive power and the full astigmatism by an unconscious (objective) refraction examination with a refractometer or the like, and to effect a self-conscious (subjective) refraction examination and final determination of the base curve by the trial lens method.
As described above, the measurements of shape of cornea and of refractive power have heretofor been carried out with different instruments and have therefore required considerable time and labor for both of the examiner and examinee.
Hard contact lens and soft contact lens have different characteristics in the correction of astigmatism. The hard contact lens can correct the corneal astigmatism to a certain degree by a correcting effect of a layer of tear formed between the lens and cornea. On the other hand, the soft contact lens is unable to correct astigmatism since it follows the curve of cornea.
Such advantage of hard contact lens can, however, appear as a drawback in certain cases. The astigmatism of an eye can be composed of corneal astigmatism and lenticular astigmatism, which might mutually compensate. In case of such an eye, the use of a hard contact lens will result in the correction of corneal astigmatism only, whereby the remaining lenticular astigmatism becomes manifested, thus aggravating the overall astigmatism. Consequently, in prescribing a contact lens, attention should be paid not only to the near- and far-sightedness but also to the astigmatism.
In prescribing a contact lens, it has been customary, as explained above, to measure the shape of cornea with an ophthalmometer or a keratometer to select the base curve and determine the degree of corneal astigmatism, then to screen the full refractive power and the full astigmatism by an unconscious refraction examination with a refractometer or the like, and to effect a self-conscious refraction examination and final determination of the base curve by the trial lens method.
In such an examination, the examiner tries a trial lens from a flatter one to a strongly curved one, according to the average radius of curvature of cornea obtained by the ophthalmometer or auto-kerato-meter and referring to a table separately prepared by the examiner, to determine a contact lens matching the examined eye. However, in such a method, the time and labor required until final determination of a suitable contact lens are considerable burdens to both of examiner and examinee.
In order to resolve such a problem, there is already known an ophthalmometer or a keratometer which which incorporates a computer that prints a lens matching the average radius of curvature of the cornea through selection from a list likewise incorporated in such an instrument. Such an instrument is further provided with a function to select a soft contact lens according to the degree of the lenticular astigmatism, but an optimum contact lens cannot be prescribed in a certain case unless attention is also paid to the remaining astigmatism as explained before.